1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) cell and, more particularly, to an LCD cell having a wider view angle for display by providing a plurality of active areas on a substrate.
2. Description of the Related Art
Liquid crystal display cells having a plurality of minute areas in each pixel, which is generally called sub-pixel domains, have been reported in Patent Publication Nos. JP-A-82-186735, JP-B-83-43723, JP-A-84-211019, JP-A-88-106624, JP-A-89-88520, JP-A-89-245223, JP-A-93-173138, JP-A-93-203951. The main object in dividing a pixel into a plurality of minute areas is to align orientations of the respective areas so that viewing angle of the areas constituting one pixel may be compensated by each other in the pixel. There exist two major technologies for aligning the orientation of a liquid crystal. One is a slanting deposition technology of an oxidized silicon film while the other is a rubbing technology in which a polyimide film is rubbed to be directed toward a certain direction, the latter of which has widely been applied due to its ease of operation. Polyimides as used in this technology are described in Patent Publication Nos. JP-A-86-47932 and JP-A-86-174725, which may be SE-7311 supplied from Nissan Chemical Corp. and AL-1051 supplied from Japan Synthetic Rubber, both of which are trademarks.
FIG. 23 of the present application (FIG. 3 of Publication JP-A-88-106624) is a cross-sectional view showing an image pixel of a liquid crystal display cell having two-orientations in each pixel.
On a first glass substrate for an active matrix, namely a TFT substrate, there are provided transparent pixel electrodes (indicated as ITO, indium-tin oxide) each having a unit size as large as a pixel, an orientation film and TFTs for driving the pixel electrodes. On a second glass substrate, namely a color filter substrate, there are also provided a transparent counter electrode (ITO) and an orientation film. Each pixel is divided into sub-pixel domains A and B.
Rubbing processings are performed on respective orientation films, in different directions in the divided sub-pixel domains A and B shown as arrows in FIG. 24. In FIG. 24, arrows written in broken lines indicate the directions for rubbing the orientation film located on the TFT substrate while arrows written in solid lines indicate the directions for rubbing the orientation film formed on the color filter substrate. On the other hand, arrows written in solid arcs shown in FIG. 24 indicate twisting directions and angles of the oriented liquid crystals between both substrates.
FIG. 25 is cross-sectional views taken along lines a-a' and b-b', respectively, of FIG. 24 showing both pre-tilt directions of the liquid crystal induced by orientation forces provided by both substrate surfaces and forces applied to the liquid crystal by the electric field acting between the substrates.
The following paragraphs 1)-3) present reasons why each pixel divided into a plurality of sub-pixel domains having different orientations of a liquid crystal attains a wider viewing angle.
1) FIG. 24 indicates that helical twist directions of the liquid crystal are the same between sub-pixel domains A and B while FIG. 25 indicates that an angle, with which the major axes of the liquid crystal in the vicinity of substrate initially intersects a substrate surface, and, which is called a "pre-tilt angle", has different rotational directions therebetween in sub-pixel domains A and B.
2) The difference in pre-tilt directions induces a difference in directions of the major axes of the liquid crystal during application of a voltage, which is called "tilt directions", as shown in FIG. 25, so that sub-pixel domains A and B compensate optical characteristics of each other within each set of sub-pixel domains in a pixel if rays of light are incident in directions other than a perpendicular direction onto the panel.
3) Viewing angle dependence of optical characteristics during application of an electric field compensated between sub-pixel domains provides optical characteristics having a less viewing angle dependence. Especially, even when a viewing angle is varied during a gradation display, a tone reversal phenomenon will not appear.
Division of a pixel into a plurality of minute areas are performed by applying a plurality of different rubbing directions to the polyimide film in the respective minute areas. Practical procedures of implementing rubbing steps for division have been reported in Publication Nos. JP-B-85-211422 and JP-B-93-203951. For instance in JP-B-85-211422, a dividing procedure of a pixel is described wherein a part of a polyimide film is masked by a photoresist film after a first rubbing step is effected for the entire area of the polyimide film, then the other part of the polyimide film is subjected to a second rubbing step and finally the photoresist film is removed. On the other hand, Publication Nos. JP-B-93-173137 and JP-B-93-203951 show another procedure for dividing a pixel wherein orientation alignment for division is performed to a polyimide film formed on a first substrate while a uniform orientation alignment is pursued to the polyimide film formed on a second substrate disposed in opposed relationship to the first substrate. In this operation, the pre-tilt angle of the liquid crystal in a vicinity of the first substrate is set equal to or larger than that of the liquid crystal in the vicinity of the second substrate, so that the orientation of the liquid crystal in the vicinity of the first substrate are dominant as compared with that of the liquid crystal in the vicinity of the second substrate. Thus, it enables a dividing orientation alignment in a fewer steps of processing.
The conventional orientation alignments for dividing a pixel as described above have problems that one orientation direction turns dominant over the other, depending on combination of rubbing orientations between the two areas.
Furthermore, a disclination line is generated on a boundary between the areas which have different orientations even when the divided orientation alignment is obtained. The disclination line shifts or deforms, especially when a driving voltage is applied to liquid crystal display cells. The shift of the disclination line is observed as an afterimage, which deteriorates performance of display cells.